The present invention relates to a method for producing rare earth metal-doped quartz glass. The method comprises the steps of:                (a) providing a blank of the rare earth metal-doped quartz glass, and        (b) homogenizing the blank by the blank being softened zone by zone in a heating zone and by twisting the softened zone along a rotation axis.        
Moreover, the present invention relates to rare earth metal-doped quartz glass which contains rare-earth metal oxide in a concentration of 0.002 to 10 mole %.
Rare earth metal-doped quartz glass is used, for example, for the production of fittings and components for semiconductor manufacture or of fiber amplifiers or fiber lasers in laser technology. In the first-mentioned field of application, the dopants bring about an improvement of the dry etching resistance of the glass material, and in the second field of application, the dopants bring about the generation of the amplification of laser radiation in the host material quartz glass.
The rare earth metals include the elements scandium, yttrium, and lanthanum and the lanthanoids.
DE 10 2004 006 017 A1 describes the production of a laser-active quartz glass doped with rare-earth or transition metals. The publication starts from an aqueous slip which contains nanoscale SiO2 particles produced by way of precipitation reaction, as well as start compounds for the dopants in the form of water-soluble hydrate compounds. After granulation, the still porous doped SiO2 granulate is put into a graphite mold and vitrified by gas pressure, sintering into a blank of doped quartz glass. The graphite mold is first heated to a sintering temperature of 1600° C. while maintaining a negative pressure. After the sintering temperature has been reached, an overpressure of 5 bar is set in the furnace and the mold is kept at this temperature for about 30 minutes. During subsequent cooling to room temperature, the overpressure is further maintained up to a temperature of 400° C.
The SiO2 blank obtained in this way is subsequently three-dimensionally homogenized. Homogenization takes place by thoroughly mixing the SiO2 blank in several directions. This results in the absence of striae and in a distribution of the refractive index that is homogeneous in three dimensions.
JP 2007-230814 A describes the manufacture of rare earth metal-doped quartz glass, with a powder mixture being sintered under vacuum or by hot pressing into a quartz glass blank, and the quartz glass blank being subsequently homogenized by twisting. This yields a rare earth metal-doped quartz glass with a rare earth metal concentration between 1.2 and 5% by wt., a low bubble content and a low OH content. The range <5×10−6 is indicated for the refractive index distribution.
WO 2005/054139 A1 describes a method for producing a blank for a component of laser-active quartz glass by providing a blank of rare earth metal-doped, laser-active quartz glass with an OH content of less than 10 ppm and by three-dimensionally homogenizing the SiO2 blank. Homogenization takes place by thorough mixing of the SiO2 blank in several directions. This results in the absence of striae and in a distribution of the refractive index that is homogeneous in three directions.
It is known from EP 1 894 896 A1 that a laser-active, rare earth metal-doped quartz glass is produced by using a mixed powder containing quartz powder and two or more types of dope elements in a total amount of 1 to 20% by wt., wherein the dope elements comprise a first dopant selected from the group of N, C and F and a second type of dopant selected from the group consisting of Mg, Ca, Sr, Ba, Sc, Y, Ti, Zr, Hf, lanthanoids and actinoids. The mixed powder is molten by way of the Verneuil method in a reducingly acting atmosphere into a quartz glass blank. Bubbles with a total cross-sectional area of around 5 mm2 are visually visible in a volume of 100 cm3 glass, and the internal transmission for visible light is 80%/cm.
U.S. Patent Application Publication No. 2005/0272588 A1 relates to quartz glass for jigs. To enhance the plasma etching resistance to fluorine-containing etching gas, it is suggested that the quartz glass should be doped with metal oxides in a concentration between 0.1-20% by wt., wherein a first dopant is selected from the group 3B of the periodic table, and a second dopant from the group Mg, Ca, Sr, Ba, Sc, Y, Ti, Zr, Hf, lanthanides or actinoids. The bubble content can be reduced by hot pressing in argon atmosphere, resulting in a total cross-sectional area of less than 100 mm2 per 100 cm3 glass.
Hot homogenization normally takes place by twisting the doped quartz glass on a glass lathe, wherein the headstock in which the quartz glass body is clamped is rotated at different speeds. Homogenization was carried out in the past such that the doped quartz glass was homogenized by way of hydrogen-oxygen burners with oxygen excess and twisted, respectively, with the doped quartz glass being well mixed.
Some rare earth metals, however, show a discoloration of the quartz glass, which hints at an unforeseeable and undesired change in the chemical composition or possibly at an inhomogeneous distribution of the dopants.
It is therefore an objective of the present invention to provide a modified method which ensures the production of rare earth metal-doped quartz glass with reproducible properties.
Moreover, it is an objective of the present invention to provide a rare earth metal-doped quartz glass which shows a low fluctuation in the refractive index and which is also distinguished by a high homogeneity of its chemical and electrical properties.